Television engineers have given considerable thought to ghost-cancelation circuitry for inclusion in television receivers that also include a display device for reproducing the television image in a form suitable for viewing by humans. Ghost images, caused by multipath reception and commonly referred to as "ghosts", are a common occurrence in television pictures that have been broadcast over the air or have been transmitted by cable.
The signal to which the television receiver synchronizes is the strongest of the signals it receives, which is called the reference signal, and is usually the direct signal received over the shortest reception path. The multipath signals received over other paths are thus usually delayed with respect to the reference signal and appear as trailing ghost images. It is possible, however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. When the receiver synchronizes to a reflected (longer path) signal, there will be a leading ghost image caused by the direct signal, or there will be a plurality of leading ghosts caused by the direct signal and other reflected signals of lesser delay than the reflected signal to which the receiver synchronizes. The parameters of the multipath signals--namely, the number of different-path responses, the relative amplitudes of the different-path responses, and the differential delay times between diffent ones of the different-path responses--vary from location to location and from channel to channel at a given location. These parameters may also be time-varying.
The visual effects of multipath distortion can be broadly classified in two categories: multiple images and distortion of the frequency response characteristic of the channel. Both effects occur due to the time and amplitude variations among the multipath signals arriving at the reception site. When the relative delays of the multipath signals with respect to the reference signal are sufficiently large, the visual effect is observed as multiple copies of the same image on the television display displaced horizontally from each other. These copies are sometimes referred to as "macroghosts" to distinguish them from "microghosts", which will be presently described. In the usual case in which the direct signal predominates and the receiver is synchronized to the direct signal, the ghost images are displaced to the right at varying position, intensity and polarity. These are known as trailing ghosts or "post-ghost" images. In the less frequently encountered case where the receiver synchronizes to a reflected signal, there will be one or more ghost images displaced to the left of the reference image. These are known as leading ghosts or "pre-ghost" images.
Multipath signals of relatively short delays with respect to the reference signal do not cause separately discernible copies of the predominant image, but do introduce distortion into the frequency response characteristic of the channel. The visual effect in this case is observed as increased or decreased sharpness of the image and in some cases loss of some image information. These short-delay, close-in or nearby ghosts are commonly caused by unterminated or incorrectly terminated radio-frequency transmission lines such as antenna lead-ins or cable television drop cables. In a cable television environment, it is possible to have multiple close-in ghosts caused by the reflections introduced by having several improperly terminated drop cables of varying lengths. Such multiple close-in ghosts are frequently referred to as "microghosts".
Long multipath effects, or macroghosts, are typically reduced by cancelation schemes. Short multipath effects, or microghosts, are typically alleviated by waveform equalization, generally by peaking and/or group-delay compensation of the video frequency response.
Since the characteristics of a transmitted television signal are known a priori, it is possible, at least in theory, to utilize such characteristics in a system of ghost signal detection and cancelation. Nevertheless, various problems limit this approach. Instead, it has been found desirable to transmit repeatedly a reference signal situated, for example, in a section of the TV signal that is currently unused for video purposes and to utilize this reference signal for the detection of ghost signals prior to arranging for the suppression of ghost signals. Typically, lines in the vertical blanking interval (VBI) are utilized. Such a signal is herein referred to as a Ghost Canceling Reference (GCR) signal; and a variety of different GCR signals have been described in patents and other technical publications.
Bessel pulse chirp signals are used in the GCR signal recommended for adoption as a standard for television broadcasting in the United States of America. The distribution of energy in the Bessel pulse chirp signal has a flat frequency spectrum extending continuously across the video frequency band. The chirp starts at the lowest frequency and sweeps upward in frequency therefrom to the 4.1 MHz highest frequency. The chirps are inserted into the first halves of selected VBI lines, the 19.sup.th line of each field currently being preferred. The chirps, which are on +30 IRE pedestals, swing from -10 to +70 IRE and begin at a prescribed time after the trailing edges of the preceding horizontal synchronizing pulses. The chirp signals appear in an eight-field cycle in which the first, third, fifth and seventh fields have a polarity of color burst defined as being positive and the second, fourth, sixth and eighth fields have an opposite polarity of color burst defined as being negative. The initial lobe of a chirp signal ETP that appears in the first, third, sixth and eighth fields of an eight-field cycle swings upward from the +30 IRE pedestal to +70 IRE level. The initial lobe of a chirp signal ETR that appears in the second, fourth, fifth and seventh fields of the eight- field cycle swings downward from the +30 IRE pedestal to -10 IRE level and is the complement of the ETP chirp signal.
The strategy for eliminating ghosts in a television receiver relies on the transmitted GCR signal suffering the same multipath distortions as the rest of the television signal. Circuitry in the receiver can then examine the distorted GCR signal received and, with a priori knowledge of the distortion-free GCR signal, can carry out a procedure known as channel characterization, in which the magnitudes, phases and occurrence times of the ghosts are determined respective to the reference signal. This is done by calculating the discrete Fourier transform (DFT) of the ghosted GCR signal and dividing the terms of that DFT by the corresponding terms of the DFT of the non-ghosted GCR signal known a priori, thus to generate the respective terms of the DFT of the channel. These DFTs are all in the time domain. The occurrence times of the ghosts and the amplitudes of their in-phase components are then used for calculating the adjustable weighting coefficients of a digital filter through which the composite video signal from the video detector is passed to supply a response in which ghosts are suppressed, which filter is referred to as a "ghost-cancelation" filter in this specification. The terms of the channel DFT are analyzed to determine the largest of them, which is replaced by zero in a modified DFT. The other terms are reversed in sign in the modified DFT, which is the desired DFT of the ghost-cancelation filter. The weighting coefficients of this ghost-cancelation filter are adjusted to approximate this desired DFT as closely as possible. The GCR signals can be further used for calculating the adjustable weighting coefficients of an equalization filter connected in cascade with the ghost-cancelation filter, for providing an essentially flat frequency spectrum response over the complete reception path through the transmitter vestigial-sideband amplitude-modulator, the transmission medium, the television receiver front-end and the cascaded ghost-cancelation and equalization filters.
The inventors have configured the ghost-cancelation filter as a cascade connection of a recursive digital filter principally used for cancelling post-ghosts and a non-recursive digital filter principally used for cancelling pre-ghosts. A recursive digital filter has an infinite impulse response, so is commonly referred to as an IIR filter. A non-recursive digital filter has an finite impulse response, so is commonly referred to as an FIR filter. One may seek to carry out adjustments to the IIR and FIR filters independently, directly relating the smaller terms of the DFT of said reception channel later in time than the largest term of the DFT of said reception channel to tap weights in the IIR filter, and directly relating the smaller terms of the DFT of said reception channel earlier in time than the largest term of the DFT of said reception channel to tap weights in the FIR filter. Directly relating the DFT terms to tap weights in the filters is a relatively simple computational procedure that was followed in the prior art when just post-ghosts or just pre-ghosts were being corrected.
Suppose, then, those portions of the channel characterization results descriptive of post-ghosts are used to adjust the filtering coefficients of just the IIR filter principally used for cancelling post-ghosts. Suppose further those portions of the channel characterization results descriptive of pre-ghosts are used to adjust the filtering coefficients of just the FIR filter principally used for cancelling pre-ghosts. When the IIR filter adjustments and FIR filter adjustments are carried out independently, ghost suppression is good when there are only post-ghosts and not too many of them. Ghost suppression is also good when there are only pre-ghosts and not too many of them.
Ghost suppression tends to be poor, however, when there are both post-ghosts and pre-ghosts of appreciable energy, even though the ghosts are few in number. The problem of too many post-ghosts and of too much differential delay between those ghosts can be solved by using an IIR filter with a greater number of taps with non-zero weighting and an increased number of programmable bulk delay devices. The problem of too many pre-ghosts and of too much differential delay between those ghosts can be solved by using a FIR filter of more complex design. When there are both post-ghosts and pre-ghosts to be suppressed, however, the problem of poor ghost suppression is not solved by dealing with the post-ghost problem and with the pre-ghost problem separately.
The inventors observe that ghost suppression is good when conditions are such that one of the cascaded filters has substantially no effect on the response of the other. The problem is that when there are both post-ghosts and pre-ghosts to be suppressed, the IIR and FIR filter responses are interactive with each other. By way of illustrating this interaction, suppose the IIR filter precedes the FIR filter in their cascade connection with each other. For each post-ghost cancelled by the IIR filter, a pre-ghost supplied to that filter will give rise to a ghost of the pre-ghost. The ghost of the pre-ghost is delayed from the pre-ghost by the same interval as the suppressed post-ghost was delayed from the predominant signal.